The planet earth is
at least 4500 million years old. The earliest signs of life date back
3000 million years, and by 600 million years ago quite complex animals
and plants existed.

The story of the backboned (vertebrate) animals -the fishes and other
forms that eventually led to humans -began about 600 to 500 million
years ago. The first definite fossil of a creature with a stiffening
notochord along its back was a Cambrian animal called Pikaea, which
was discovered in 560- million-year-old rocks in Canada. The oldest
complete fish fossil so far found came from central Australia, and was
ajawless, armoured fish called Arandaspis, which dates from the
early Ordovician period. This small filter-feeding or mud- grubbing fish
lived in shallow coastal waters over 480 million

years ago.

Pteraspis

Thelodus

Two
of the earliest backboned animal~ on earth. These ancient fishes,
which lived in seas that covered the earth between 350 and 440
million years ago, had primitive, sucking mouths

In the next
100 million years all the major fish groups that are found on earth
today evolved, although their origins and relationships to one another
arestill uncertain. In the
Silurian period -some 400 million years ago -the jawless fishes were the
most numerous, although primitive bony and cartilaginous jawed fishes
had begun to appear at about that time. Fossils of the so-called 'spiny
sharks' -which had large fin spines -andof
the earliest true sharks have also been found in Silurian rocks. By the
Devonian period -about 30 million years later -fishes had diversified
and spread into all parts of the world. By the end of the Devonian, some
25 million years further on, certain fishes had moved onto land. From
these first amphibians evolved the four-legged animals, including
reptiles such as dinosaurs, and eventually mammals.

The early evolutionary history of sharks and shark-like fishes is

still poorly understood. Until
recently scientists thought that there were no shark fossils to speak of
in rocks older than those from the Middle Devonian. Now, however, itis certain that sharks did not
appear suddenly at that time -itwas
just that researchers were looking for the wrong kind of evidence.
Microscopic examinationof
ancient sediments has revealed fossilized remains of sharks which may
push their origins back at least 50 million years further.

Traditionally; sharks have been regarded as primitive vertebrates -
so-called 'living fossils' -but recent work suggests that" they are
highly specialized. Their complex biology ranks them with birds and

mammals as highly evolved

Creatures.
Even Even Devonian sharks, which were once considered to be the most
primitive, are now thought to have been relatively specialised - with a
longer evolutionary pastthan was once supposed.

The evolution of sharksremains unclear partly because it has been difficult to
analyse- particular characters -such as the shapes ofbraincases or
finstructures -of modern sharks,which are an extremely diverse group,
adapted to many habits and habitats. Any analysis of fossilsharks
has had to rely almost solely

Fossil shark teeth embedded in a piece of
Miocene limestone from Victoria in Australia. Detailed examination of
such remains has enabled scientists to piece together the complex story
of how life evolved on earth.

on hard parts, such as
teeth. Other features such as fins are only rarely preserved. Much more
needs to be known about ancient sharks before any assessment can be made
as to which characters were 'primitive: and which were 'advanced~

All that is known about ancient sharks
and their evolution has been gleaned from the fossil record.Fossil
remains of sharks have been known for many centuries, although their
true nature was not always recognized. Until the 17th century many
scholars regarded such fossils as sports of nature, and thoughtthat fossilized shark's teeth were bird's or snake's
tongues. Some large fossil teeth, called glossopteris (literally
tongue-stones), were used as amulets to ward off evil and to protect
against poisoning. It was not until about 150 years ago that the study
of fossils, paleontology; became a science, and the ancient remains of
plants and animals were systematically classified.

Sharks are rarely found
as complete fossils because their skeletons are made of cartilage.
Normally only the hard parts, such as teeth, scales and fin spines, are
found. However, under certain special conditions, complete fossil sharks
are preserved, and these provide scientists with vital information. One
such deposit,found
last century in Upper Devonian Cleveland shales from the USA, yielded
entire shark carcasses which had been preserved in a bacteria-free
environment so that even muscle and kidney tissue could be examined in
the rock.

The very earliest signs of sharks are minute
fossil scales and teeth which are found in rocks from the late Silurian to early
Devonian

period
{around 400 million years ago). It becomes more and more difficult, however, to
identify shark scales in older rocks because they closely resemble those fromjawless fishes called thelodonts, which lived at the same
time. Only microscopic differences separate shark and the lodont scales, and the
two kinds seem to become more and morealike
the further one goes back.

A similar problem exists with ancient shark teeth, which did not seem to be
present in rocks older than those from the mid-Devonian. It now seems that the
reason for

this was that
scientists were not looking in the right places, and that early shark teeth were
often very small. In 1986 teeth were found in Lower Devonian rocks from Spainwhich belonged to a group
of sharks called Xenacanthids.

What, then, were the origins of the shark-like fishes? One possibility is
that different cartilaginous fishes (the group which includes sharks, skates,
rays and chimaeras) evolved from placoderms (now extinct, bony- plated jawed
fishes). Alternatively the placoderms and the cartilagenous fishes might all
have shared a common ancestor at some time in the early Silurian period, some
430 million years ago. A third possibility is that the cartilaginous fishes and
a group of primitive jawless fishes, such as the

lodonts, both had a
common ancestor. Thelodonts
(now extinct) had a skeleton made of cartilage and were covered with scales,
which may

have lined their mouths as well. They also had paired fins and eight pairs of
branchial structures for supporting their gills. One region where the

lodonts evolved
rapidly was around Tuva, in Siberia, which is also where
the oldest known shark scales have been found.

The common ancestor of sharks and the

lodonts (a fossil of
which is yet to be found) would probably. have
been a small fish with a long slender body; one dorsal fin, no fin spines,
paired pectoral fins and at least seven pairs of gill supports. Its mouth would
have been either at the front of its body; or slightly beneath it, and the
creature would have been covered with small scales which varied in size and
shape according to their position on its

ANCIENT ANCESTORS OF MODERN SHARKS

Cladoselache

The earliest complete fossil sharks so far found belong to this genus They
have very distinctive three-pointed teeth which first appear in rocks from fhe
mid-Devonian period, and gradually disappear by the Permian period Specimens
range from 05to 2 m (15to6 ft) in length

Hybodus &
Stethacanthus

The first hybodont sharks appeared during the

Carboniferous
period and members of this grouponly
died out about 65 million years ago. at theend
of the Cretaceous period Specimens of Hybodus grew to around 25 m (75 ft)

The story of how sharks developed from these ancestral forms is complex and
confused. Scientists do not have a a set of fossil remains which show a smooth
transition from species to species, connecting ancient forms to their modern
descendants. Instead they have some isolated pieces from an immensely complex
jigsaw. Some fossils are of evolutionary experiments which led nowhere and eventually
became extinct. Others may have features which seem to explain gaps in our
knowledge, but also have other features which raise more questions. Some new
discoveries provide little information at present, but may

become
vital when other pieces of the jigsaw are in place. Many theories exist to
explain theevidence that is available, and these are
constantly being modified as new information comes to light.

Although there is evidence of earlier sharks, the first complete fossils of
shark-like fishes have been discovered in mid-Devonian rocks. Most frequently
found are members of the genus Cladoselache, streamlined fish that grew
to a length of about two meters (6 ft). Fortunately; complete specimens of Cladoselache
have been preserved

in the remarkable Cleveland shales,
so quite a lot is known about them. They had five pairs of gill slits, a fin
spine and all the same fins as modern sharks, except for an analfin. These spines, which become more common and elaborate
in later sharks, and which still persist in some species today; were positioned
in front of the dorsal fins and acted as cutwaters. Cladoselache had
distinctive teeth with a largecentral cusp flanked by
sever-al smaller points, and apparently they lived on small fish -the remains of
which have been found in the stomachs of fossilized specimens. These sharks are
not nowconsidered to be the main line leading to the
modern species.

One important feature, however, was missing from Cladoselache, and
most of its relatives in Devonian

seas -claspers. These
copulatory devices, which all modern sharks have, are only found in the

Shark teeth, together with scales and dorsal spines, are the three parts of
sharks most often preserved in rocks. Both crushing teeth, for eating shellfish,
and pointed teeth, for eating flesh, are included in this collection.

ONE OF NATURE'S STRANGER
EXPERIMENTS

Perhaps the most
curious of all ancient generated at the front of the creature's jaw
teeth are those from the shark-Like creature Helicoprion, which
first appeared during the Carboniferous period, some 345 millionyears
ago. Fossil teeth in the form of spirals or whorls are found in rocks
worldwide and gave rise to many fantastic reconstructions animal's life.
Almost nothing is known about before partially complete fossil specimens
how these strange creatures lived, andwere
found which enabled researchers to see how the teeth might have fitted
inside the fish's jaws.

Fossil teeth (left)
from Helicoprion.

Reconstruction
(below) shows
how teeth may have fitted into the jaw.

stethacanthids and a few other groups
of ancient sharks that lived at the same time as Cladoselache.In
the Devonian period a new group of sharks became common - the hybodonts. These
became the dominant group during theMesozoic
era (from about 220 to 65 million years ago), when nearly all other shark groups
died out. Although they were not as streamlined as modern oceanic sharks, and
were probably not as accomplished as swimmers, they were nevertheless an advan~e
on Cladoselache. A typical genus of this group is Hybodus which
was found all around the world from the Triassic period to the mid- Cretaceous.
This shark grew to at least 2.5 m (7.5 ft) in length, and had a blunt snout with
an elongated body The arrangement of its teeth - with sharp pointed ones at the
front and blunt ones at the back -seems to suggest that it caught swimming

The curious teeth of
a frilled shark Chlamydosalachus anguineus -one of the so-called
'primitive' sharks, which closely resembles some extinct species. This
shark has some 300, three-pointed teeth, arranged in 27 rows, to provide
it with about 1000 hooks with which to grasp the fish it eats.

prey as well as eating shellfish,such
as molluscs.

Although sharks and shark-like fishes
have a long history; the modern sharks Cneoselachians) did not rise to dominance
until after the Jurassic period, when, for some reason that is not yet clear,
many of the more ancient forms had become extinct. Some Jurassic sharks are
closely related to modern sharks, and this gives many present-day shark families
histories which stretch back for 135 million years or more. The skates and rays,
another group of cartilaginous fishes, also appeared in the mid-Jurassic, but
they did not really come into their own until the Tertiary period, between 65
and 2 million years ago, when they were able to exploit a dramatic rise in the
numbers of bivalve shellfish in the oceans.

The fossil record of modern sharks is fairly good, but again it normally
consists only of hard parts

such as
teeth and scales. The appearance and relationships of present-day groups are
well understood compared with the situation in the Paleozoic and Mesozoic eras.
Oceans in the Cretaceous period (140 to 65 million years ago) were dominated by
goblin, sand, probable, nurse, cow and angel sharks. Early sawfishes appeared
and later evolved into the modern sawsharks. At the beginning of the Tertiary
period (about 65 million years ago) thegray
nurse sharks (odontaspididae) were found in large numbers. All modern forms of
sharks were present in seas of the Miocene epoch (25 to 5 million years ago),including the giant Carcharodon
megalodon, perhaps the most awesome of all sharks, now extinct.

Many researchers are working on
the problems presented by ancient sharks, and recent discoveries have included
some of the oldest shark fossils found.

Among international research has been work in Germany which
has concentrated on examining the fine structure of shark teeth and scales, and
has revealed that the outer enamel on many teeth has threelayers
-a feature which can be usedto identify certain groups of
sharks. Other work, in the USA, has been delving into the nature of the
supposedly primitive modernspecies -the frilled shark Chlamydoselachus
anguineus, the Port Jackson shark Heterogonous portusjackson and the
sixgill and sevengill sharks (family hexanchidae: -in an attempt to understand
their relationships to other modern sharks, and to uncover any links with
earlier fossil forms.

One particularly important recent discovery has been the
recognition of a new class of fossil sharks called iniopterygians.

Remains of this strange group have been found in Upper
Carboniferous rocks from North America which are 300 million years old.

Examination has shown that these creatures are a 'missing
link: and that they combined certain characteristics of both elasmobranches
{sharks, skates and rays) and holocephalians (chimaeras orrattails).
The life styles of these fishes remain uncertain, although their complex tooth
plates seem to suggest that they lived on shellfish.

There has been an upsurge of interest in fossil sharks in
recent years as more information has become available from new fossil
discoveries, especially in the southern continents, and from a microscopic
examination of existing fossil remains. In many cases these discoveries have
helped scientists to understand some of the finds made last century~ There are
still few definite answers about the origins and relationships of all the known
fossil, and some modern, sharks but progress is being made.

ATRUE MONSTER OF
THE DEEP

Some 12 000 years ago, at about the time when
mammoths became extinct, humans who ventured into the sea faced the
possibility of meeting the largest fish- eating shark that has ever lived -a
close relative of the great white.

Estimates vary for the length of Carcharodon
megalodon -as this monster has been named -but the most realistic seem
to agree on a figure of between 12 and 15 m (40 and 50 ft), with a weight of
around 20 tonnes.

Teeth from this giant shark -some measuring
up to 152 mm (6 in) in length, and weighing 340 g (12 oz) -are"commonly
found in Palaeocene sediments fromaround
the world. Many have also been dredged from the sea floor.

Nobody can be sure exactly when megalodon became
extinct, and some optimists have even claimed that isolated specimens may
still exist in the ocean depths today. Most scientists, however, dismiss
this possibility out of hand.

Two teeth from
the giant extinct shark Carcharodon megalodon, compared with that of
a great white. The fossil teeth are large -probably as large as any found -
and came from a shark that
must have

been at least 12
m (40 ft) long. The great white to which the single tooth belonged
was around 3 m (9.8 ft) long. The enamel height on the largest
megalodon tooth ever found is 115 mm (4.53 in),
compared with a maximum for a great white of around 70 mm (2.8 in).